                    [:kg-per-m3 :lb-per-cu-ft] #(/ % (factors [:lb-per-cu-ft :kg-per-m3]))
                    [:ft-lb :J]                #(* % (factors [:ft-lb :J]))
                    [:J :ft-lb]                #(/ % (factors [:ft-lb :J]))
-                   [:lbf :N]                  #(* % (factors [:lbf :N]))
-                   [:N :lbf]                  #(/ % (factors [:lbf :N]))
                    [:ft :m]                   #(/ % (factors [:m :ft]))
                    [:m :ft]                   #(* % (factors [:m :ft]))
                    [:kg :lb]                  #(* % (factors [:kg :lb]))

(defn convert-units-f [from to]
  (let [factors   {[:lb-per-cu-ft :kg-per-m3] 16.0185
                   [:ft-lb :J]                1.35582
                   [:lbf :N]                  4.44822
                   [:m :ft]                   3.28084
                   [:kg :lb]                  2.20462262185
                   [:hp :W]                   745.69987158227022

        measurement (generate-measurement-f unit)]
    [measurement (measurement-as-f measurement (first (remove (partial = unit) units)))]))

(generate-pair #{:lbf :N} rand-force-unit typical-force force-as)
